Extracts of pancreas and placenta have been found to contain, under special conditions, material which appears to be beta endorphin by virtue of a) radioreceptor assay using H3-Naloxone and b) radioimmune assay using antisera prepared against synthetic human beta endorphin. Both tissues contain a beta endorphin with a molecular weight of approximately 4800 daltons rather than the 3400 dalton material from the pituitary. This activity will pass through a 5,000 dalton Amicon ultrafilter and yet be excluded by G-25! It will be held back slightly behind the void volume of P-6 exclusion column. The larger molecular weight presumably results from an extra 12 or 13 amino acids probably on the C-terminal end of the molecule. It may well be that this extra peptide is what eliminates the usual respiratory depression characteristic of morphine and to a lesser extent beta endorphin itself. We propose to purify to homogeneity and characterize in terms of structure in vivo this large molecular weight beta endorphin from one or both tissues. It is possible that the pancreas in particular can be obtained sufficient quantities to permit a significant amount of this hormone to be prepared commercially at a considerably more economic price.